A known clutch device is generally arranged at a drive system for a vehicle. The clutch device transmits and blocks a driving force of a drive source of the vehicle between an input shaft connected to the drive source and an output shaft connected to driving wheels of the vehicle. In such clutch device, plural first clutch plates arranged at the input shaft and plural second clutch plates arranged at the output shaft are positioned in an alternating manner with one another. A hydraulic pressure of oil is applied to a hydraulic chamber or the hydraulic pressure is released from the hydraulic chamber, thereby pressing the plural first and second clutch plates against one another. Accordingly, the first and second clutch plates arranged side by side are engaged with (connected to) one another. Consequently, the input shaft and the output shaft are rotatably connected to each other.
In the clutch device configured as described above, a driver of the vehicle may have uncomfortable sensations because of a delay in the engagement of the first and second clutch plates of the clutch device. Further, the first and second clutch plates may be worn or stuck to one another because of slippage therebetween due to the incomplete engagement. Additionally, for example, a delay in disengagement between the first and second clutch plates of the clutch device occurs or the clutch device is not released from the engaged state; therefore, the vehicle may not be appropriately brought into motion. Such defective phenomena are caused by a centrifugal force generated by the oil that should be essentially discharged from the hydraulic chamber but remains in the hydraulic chamber. For example, the hydraulic chamber is brought into rotation in a state where the oil remains in the hydraulic chamber. At this time, the centrifugal force is generated by the oil remaining in the hydraulic chamber, thereby biasing or moving a piston defining the hydraulic chamber. A canceller chamber for biasing the piston in an opposite direction of the moving direction of the piston is arranged in the clutch device disclosed, for example, in each of JP2010-91083A and JP2007-155077A (hereinafter referred to as References 1 and 2).
The clutch device disclosed in Reference 1 is a normally closed clutch. According to the clutch device according to Reference 1, when a rotary shaft is not rotating, a hydraulic oil (corresponding to the oil) is sealed in the canceller chamber by an oil discharge hole being in a closed state. Therefore, even when the rotary shaft starts rotating, a centrifugal hydraulic pressure generated by the oil remaining in a hydraulic chamber is cancelled by a centrifugal hydraulic pressure of the canceller chamber. Consequently, the clutch device is not unexpectedly brought into a connected state and is therefore maintained in a disconnected state. Meanwhile, when the rotary shaft is rotating, the oil is discharged from the canceller chamber through the oil discharge hole being in an opened state. Accordingly, the piston moves toward the canceller chamber without being affected by the centrifugal hydraulic pressure, therefore bringing the clutch device into the connected state.
The clutch device disclosed in Reference 2 is a normally closed clutch. According to the clutch device according to Reference 2, in order to bring the clutch device into a disconnected state, a hydraulic pressure of a hydraulic oil (corresponding to the oil) is applied to a hydraulic pressure canceller chamber (corresponding to the canceller chamber), thereby canceling a centrifugal hydraulic pressure generated by a centrifugal hydraulic pressure caused by the oil remaining in a hydraulic chamber. Accordingly, the clutch device is smoothly brought into the disconnected state. Meanwhile, in order to bring the clutch device into a connected state, the hydraulic pressure is applied to the hydraulic chamber, thereby moving a clutch piston toward the canceller chamber. At this time, the hydraulic pressure of the canceller chamber is released therefrom through an oil discharge passage; therefore, the clutch piston is brought into the connected state while not being affected by the hydraulic pressure of the canceller chamber.
However, the clutch device according to Reference 1 is not provided with a mechanism that surely and promptly brings the clutch device into the connected state. In addition, the clutch device according to Reference 1 includes a partition wall at which a check ball is arranged. Accordingly, the clutch device has a complex configuration, resulting in an increase of costs for the clutch device. Likewise, the clutch device according to Reference 2 is not provided with a mechanism that surely and promptly brings the clutch device into the connected state. In addition, for example, in a state where a drive source of the vehicle is in a stopped state, the clutch device according to Reference 2 is configured so that the oil is released from the canceller chamber. Therefore, for example, when the drive source is returned from the stopped state into operation, a predetermined time is required until the canceller chamber is filled with the oil. Accordingly, the centrifugal hydraulic pressure generated by the oil remaining in the hydraulic pressure may not be cancelled by the centrifugal hydraulic pressure of the canceller chamber until the predetermine time elapses. Consequently, the clutch device may not be promptly brought into the disconnected state when the drive source is suddenly switched from the stopped state into operation at high speed.
A need thus exists for a clutch device for a vehicle, which is not susceptible to the drawbacks mentioned above.